The art of surveying involves the determination of unknown positions, surfaces or volumes of objects or setting out of known coordinates using angle and distance measurements taken from one or more positions. In order to make these measurements, a surveying device frequently used is a distance measuring instrument with an integrated distance and angular measurement of the type which is called a total station, i.e. with combined electronic, optical and computer techniques. A total station is furthermore provided with a computer or control unit with writable information for measurements to be performed and for storing data obtained during the measurements. Preferably, the total station calculates the position of a target in a fixed ground-based coordinate system.
In, for example, WO 2004/057269 by the same applicant, such a total station is described in more detail. Generally, a measuring or surveying instrument of the type commonly referred to as a total station or theodolite includes, with reference to FIG. 1, a movable unit 20 including optical equipment indicated with a lens 30, for example, a camera for capturing a field of view and an identified target point within the field of view. The movable unit 20 is supported in a housing 40 in such manner that it is pivotable relatively to the housing 40 around a first axis 50, as indicated by double arrow 60. The first axis 50 also may be referred to as the trunnion axis. The housing 40 is rotatable relatively to a base 80 around a second axis 90 as indicated by double arrow 100. The housing may also be referred to as the alidade portion 40. Thus, by rotating the movable unit 20 around the two axes 50 and 90 the movable unit 40 can be oriented in any desired position for the purpose of carrying out an intended surveying operation. When performing distance measuring or surveying tasks using a distance measuring total station, for example, at a work site, naval work site, a construction work site or a mining work site, a high degree of accuracy is required, with acceptable tolerances measured in arc-seconds for angles and millimetres for distance. The trunnion axis 50 is, in an ideal case, always perpendicular to the second axis 90. Furthermore, the second axis 90 is, in the ideal case, vertical. Unfortunately, however, there will often be deviation in the real case, which may influence or affect the accuracy and reliability of the measurements-to-be-performed. In order to determine the deviation between the second axis 90 and a true vertical line (defined by the gravity field) 120, illustrated in FIG. 2, a tilt sensor is arranged in the housing 40 of the total station. The tilt sensor is adapted to determine the relationship between the vertical axis of the instrument (i.e. the total station) and the true vertical line. Thereby, it is possible to compensate for the deviation. In prior art total stations, a common tilt sensor comprises a vessel filled with a fluid. The fluid is free to move so that if the tilt sensor is not level, the detection device is not orthogonal to the surface. The detection device may consist of a light emitter, for example, a LED emits a light beam which is reflected against the surface of the fluid and the reflected beam is directed to a light sensor, for example, a CCD-chip. The CCD-chip measures where the light centre of the incident light beam is located and thus it is possible to determine the deviation between the second axis 90 and the true vertical axis by determining the deviation of the measured light centre of the incident light beam and a reference point.
However, the prior art tilt sensor is rather slow due to the fact that after a movement of the housing 40, a settling time is required to allow for the fluid to orient since it will swish about in the vessel during a period of time after a movement. Thus, a damping time is required before the deviation measurements can be initiated and a deviation determination can be delivered.
Other types of tilt sensors of the prior art, for example, a tilt sensor comprising a balance plate arranged in proximity to capacitive sensors where the distance between respective sensor and the plate is measured by means of the capacitance or a tilt sensor including an element suspended by wires are also associated with the same disadvantage, i.e. that the movement influences the sensor such that a damping time is required.
Accordingly, there is a need within the art of an improved tilt sensor that is capable of providing faster deviation measurements.